Recovery of C2 + hydrocarbons by plural stage rectification and first stage dephlegmation

Abstract

A process is provided for the separation and recovery of a major portion of the C 2 + hydrocarbon content of a feed gas comprising hydrogen, methane, ethylene, and ethane. The feed gas is introduced to a dephlegmator to effect its separation into a vapor stream and a condensate stream. The condensate stream which is rich in C 2 + hydrocarbons is passed to a demethanizer column where it is fractionated into an overhead methane-hydrogen stream and a bottoms product ethylene-ethane+ stream.

We claim: 
     
       1. A method of separating and recovering a major portion of the C 2  + hydrocarbon content of a feed gas comprising hydrogen, methane, ethylene and ethane, which comprises: a. introducing said feed gas into a dephlegmation zone comprising a plurality of indirect heat exchange passages through which at least a portion of the feed gas passes in an upward direction and is cooled by refrigerant means provided in indirect heat exchange with said passages, as said gas flows upward it partially condenses on the walls of said passages forming a reflux liquid such that interaction takes place between the upward flowing gas stream and the stream of cooler liquid flowing downward and said liquid gradually becomes enriched with the C 2  + hydrocarbons of the feed gas;   b. cooling at least a portion of said feed gas in the dephlegmation zone by indirect heat exchange to form a condensate stream containing the major portion of the C 2  + hydrocarbons in the feed gas and a vapor stream containing the balance of the uncondensed material from the feed gas;   c. withdrawing the vapor stream from the dephlegmation zone;   d. passing the condensate stream from the dephlegmation zone to a rectification zone; and   e. withdrawing from the rectification zone a substantially C 1  and lighter stream and a C 2  + hydrocarbon stream, whereby the energy requirements for the refrigeration utilized in separating the C 2  + hydrocarbons from the C 1  and lighter components are low.     
     
     
       2. The method in accordance with claim 1, wherein the vapor stream of step (c) is at least partially condensed and utilized in the indirect heat exchange occurring in the dephlegmation zone. 
     
     
       3. The method in accordance with claim 2, wherein the condensed portion of the vapor stream is separated from the uncondensed portion and said condensed portion is expanded, partly evaporated, warmed in indirect heat exchange to effect the partial condensation of the vapor stream from the dephlegmation zone, thereby becoming a first dephlegmation zone refrigerant stream and is then utilized in the indirect heat exchange occurring in the dephlegmation zone. 
     
     
       4. The method in accordance with claim 3, wherein the substantially C 1  and lighter stream is withdrawn from the rectification zone as a vapor and is then at least partially condensed and admixed with the first dephlegmation zone refrigerant stream and said admixed stream is then utilized in the indirect heat exchange occurring in the dephlegmation zone. 
     
     
       5. The method in accordance with claim 4, wherein said admixed stream is passed from the dephlegmation zone seriatim through at least one turbine and at least one compressor, said turbine and compressor being interconnected whereby energy extracted from the admixed stream as it expands in the turbine is used to drive the compressor, and refrigeration provided by the turbine expansion is utilized to at least partially condense the substantially C 1  and lighter vapor stream withdrawn from the rectification zone. 
     
     
       6. The method in accordance with claim 5, wherein the feed gas is partially condensed prior to being introduced to the dephlegmation zone and separated into a first vapor stream which is fed to the dephlegmation zone and a second condensate stream which is admixed with the condensate stream withdrawn from the dephlegmation zone, said admixed condensate stream serving as a feed to the rectification zone. 
     
     
       7. The method in accordance with claim 4, wherein the refrigeration provided by the turbine expansion is utilized in the indirect heat exchange occurring in the dephlegmation zone. 
     
     
       8. The method in accordance with claim 7, wherein the feed gas is partially condensed prior to being introduced to the dephlegmation zone and separated into a first vapor stream which is fed to the dephlegmation zone and a second condensate stream which is admixed with the condensate stream withdrawn from the dephlegmation zone, said admixed condensate stream serving as a feed to the rectification zone. 
     
     
       9. The method in accordance with claim 4, wherein at least a portion of the refrigeration provided by the turbine expansion is utilized to at least partially condense the substantially C 1  and lighter vapor stream withdrawn from the rectification zone and the remaining portion of the refrigeration is utilized in the indirect heat exchange occurring in the dephlegmation zone. 
     
     
       10. The method in accordance with claim 9, wherein the feed gas is partially condensed prior to being introduced to the dephlegmation zone and separated into a first vapor stream which is fed to the dephlegmation zone and a second condensate stream which is admixed with the condensate stream withdrawn from the dephlegmation zone, said admixed condensate stream serving as a feed to the rectification zone. 
     
     
       11. The method in accordance with claim 1, wherein the substantially C 1  and lighter stream is withdrawn from the rectification zone as a vapor and is then at least partially condensed and utilized in the indirect heat exchange occurring in the dephlegmation zone. 
     
     
       12. The method in accordance with claim 1, wherein the feed gas is partially condensed prior to being introduced to the dephlegmation zone and separated into a first vapor stream which is fed to the dephlegmation zone and a second condensate stream which is fed to the rectification zone. 
     
     
       13. The method in accordance with claim 1, wherein the vapor stream of step (c) is at least partially condensed, separated into a third vapor and a third condensate, and the third condensate is introduced to the rectification zone. 
     
     
       14. The method in accordance with claim 13, wherein the substantially C 1  and lighter stream is withdrawn from the rectification zone as a vapor and is at least partially condensed and expanded to form a second refrigerant, and a substantially C 1   and lighter liquid stream is withdrawn from the rectification zone and is expanded, partly evaporated, warmed in indirect heat exchange while being utilized in the partial condensation of the vapor stream from the dephlegmation zone, and is then admixed with said second refrigerant to form a combined refrigerant stream which is utilized in the indirect heat exchange occurring in the dephlegmation zone and then recovered. 
     
     
       15. The method in accordance with claim 14, wherein prior to the expansion of the substantially C 1  and lighter liquid stream withdrawn from the rectification zone it is cooled in indirect heat exchange with the third condensate, said third condensate being warmed prior to being introduced to the rectification zone. 
     
     
       16. The method in accordance with claim 15, wherein the feed gas is partially condensed prior to being introduced to the dephlegmation zone and separated into a first vapor stream which is fed to the dephlegmation zone and a second condensate stream which is fed to the rectification zone. 
     
     
       17. The method in accordance with claim 13, wherein the substantially C 1  and lighter stream is withdrawn from the rectification zone as a vapor and is at least partially condensed and expanded to form a second refrigerant, and a substantially C 1  and lighter liquid stream is withdrawn from the rectification zone and is expanded and partly evaporated and then admixed with said second refrigerant to form a mixed refrigerant stream which is warmed in indirect heat exchange while being utilized in the partial condensation of the vapor stream from the dephlegmation zone, and is then utilized in the indirect heat exchange occurring in the dephlegmation zone and then recovered. 
     
     
       18. The method in accordance with claim 17, wherein prior to the expansion of the substantially C 1  and lighter liquid stream withdrawn from the rectification zone it is cooled in indirect heat exchange with the third condensate, said third condensate being warmed prior to being introduced to the rectification zone. 
     
     
       19. The method in accordance with claim 18, wherein the feed gas is partially condensed prior to being introduced to the dephlegmation zone and separated into a first vapor stream which is fed to the dephlegmation zone and a second condensate stream which is fed to the rectification zone.